Logic Gates What are logic gates? In the binary lesson, we - PowerPoint PPT Presentation

Logic Gates

What are logic gates? • In the binary lesson, we discussed the switches inside a computer Logic gates are the switches that turn ON or OFF • depending on what the user is doing! • They are the building blocks for how computers work.

What are logic gates? • Logic gates turn ON when a certain condition is true, and OFF when the condition is false – They check whether or not the information they get follows a certain rule • They either spit out the answer true (ON) or false (OFF) • Remember: – True= ON = 1 – False = OFF=0

Let’s do an example! • Let’s say a certain logic 5 gate needs to determine if ON Logic two numbers are equal (they are Gate 5 equal) The rule would be “is equal” • • If the two input numbers are equal, it will go into its ON position, indicating true 5 If they are not equal, it will • OFF Logic go into its OFF position, (they are Gate 6 not equal) indicating false

Let’s do an example! • But we learned before that computers only think of things 1 in terms of ON and OFF, which (ON) 1 Logic to them is 1 and 0 1 Gate (ON) • So a computer wouldn’t take (ON) an input of 5 or 6 – all of the information need to be 0s and 1s Reminder : Input refers to the • 1 information you give the logic (ON) 0 Logic gate, and output refers to what 0 Gate it spits out! (OFF) (OFF) Let’s try this example again, • keeping this rule in mind!

Types of Logic Gates! • Major logic gates: NOT, AND, OR, and XOR • There are also other ones, such as NAND, NOR, and XNOR that we’re not going to cover. This is called Boolean logic • In a circuit schematic each • logic gate is represented by a different picture, like the ones shown below.

NOT • NOT is the most simple logic gate. • All it does is take in an input that is either ON or OFF and spits out the opposite. So for a 1 it will give a 0, and for a 0 it will give a 1. • Another name for a NOT gate is inverter, because it inverts • (makes opposite) the input

AND • Unlike NOT, AND needs two inputs • It only turns on when both inputs are ON If only one input is on, it spits out OFF • If both inputs are off, it spits out OFF •

AND Truth Table • A convenient way to visualize the outputs for the logic gates is through a truth table • The truth table depicts the gate’s response to each possible set of inputs Input 1 0 1 0 0 0 Input 2 1 0 1 Output

OR • OR also needs two inputs • OR needs one input to be ON for it to spit out ON It is also ON when both inputs are ON • It is OFF when both inputs are OFF •

OR Truth Table Input 1 0 1 0 0 1 Input 2 1 1 1 Output

XOR • XOR is the short way to say “Exclusive OR” • Like OR, XOR also only needs one input to be ON for it to spit out ON But unlike OR, when both inputs are ON, XOR spits • out OFF • It is also OFF when both inputs are OFF

XOR Truth Table Input 1 0 1 0 0 1 Input 2 1 1 0 Output

Stacking Logic Gates! • An output of one logic gate can be an input to another logic gate. • This creates trees of gates that depend on each other.

Let’s Do an Example! 1 1 0

Example 2! 1 AND 1 0 0

Example 3! 1 1 1 AND 0 1 1 0

Logic gates actually look like weird bugs in real life! However, the diagrams we use are easier to understand

Any Questions??